A power converter (also referred to as a “voltage regulator”) is a power supply or power processing circuit that converts an input voltage waveform into a specified output voltage waveform. DC-DC power converters convert a direct current (“dc”) input voltage into a dc output voltage. Controllers associated with the power converters manage a power converter operation by controlling conduction of power switches employed therein. Generally, the controllers are coupled between an input and output of the power converter in a feedback loop configuration (also referred to as a “control loop” or a “closed control loop”).
Typically, the controller measures an output characteristic (e.g., an output voltage, an output current, or a combination of an output voltage and an output current) of the power converter, and based thereon, controls conduction of a power switch of the power converter. For example, in a power converter employing a linear regulator, i.e., a power converter circuit topology wherein a power switch is interposed between an input and an output of the power converter, the controller controls conductivity of the power switch within the linear region of the power switch's response curve, to regulate an output voltage of the power converter. In a power converter employing a switched-mode circuit topology, the controller controls a duty cycle of the power switch to regulate an output voltage. The duty cycle is a ratio represented by a conduction period of a power switch to a switching period thereof. Thus, if a power switch conducts for half of the switching period, the duty cycle for the power switch would be 50 percent. Additionally, as the voltage or the current for a load system dynamically changes due to a change in a system operating condition, the controller should be configured to dynamically increase or decrease the duty cycle of the power switches therein to maintain an output characteristic such as an output voltage at a desired value. A controller should also be responsive to an overload condition of the power converter, such as a short circuit coupled across an output thereof. A consideration for the design of such controllers is its response time for an overload condition, and overshoot of the output voltage that may produce an overvoltage condition when the short circuit is removed.
Turning now to FIG. 1 illustrated is a schematic drawing of a conventional standby power converter including a comparator to provide rapid shutdown of an internal pass transistor upon detection of an output voltage overvoltage condition. The circuit illustrated in FIG. 1 includes a “linear regulator” formed with p-type power MOSFET Q3 that controls the output voltage Vout at an output terminal of the regulator at a regulated voltage level. The conventional design illustrated in FIG. 1 is well understood in the art and will not be described further herein in the interest of brevity.
If overvoltage protection is not included in the circuit, design limitations of a conventional design may interact to produce a very high over-shoot condition for the regulator output voltage. A standby regulator is often used to supply a regulated voltage to a sensitive load circuit element such as a microprocessor control unit, and a high output over-shoot voltage can damage the load.
Systems, particularly portable systems such as notebook computers are frequently configured to operate in a standby mode to conserve energy delivered by a power source, which may be a battery. Such systems are frequently required to operate for an extended period of time in a standby mode, and minimizing power dissipation in the standby mode is often a key system characteristic that enables extended operation from a power source such as a battery. An overvoltage comparator that may be included in the circuit to detect an over-shoot voltage generally consumes a significant level of standby current.
Providing low power dissipation in a standby mode for a power converter controller is a key success factor for such systems in the marketplace. Present system designs address this need with circuit components that draw a low level of bias current with attendant cost and performance compromises. The design of an improved arrangement to control a power converter, particularly in a standby mode, would address an unresolved application need.